Time delay fuse element



INVENTORS Filed Feb. 12, 1958 TIME DELAY FUSE ELEMENT E. S. SUTTON, JR., ET AL March 20, 1962 Y J E M m T Aw T T SRE p A A 0 K TH m B N N R RHMA E 3,625,?95 Patented Mar. 29, 1962 3,ili25,795 Tilldll DELAY ELEMENT Ernest Show Sutton, its, Nevvarlr, Del, high T. Reilly and George Floridian, Elhton, Md, and Arnold Erwin, Wilmington, Deb, assignors to Thiolsol Chemical (Corporation, Trenton, NJL, a corporation of Delaware IP-iled Feb. 12. 1958, Ser. No. 715,345 Claims. (til. 102-435) This invention relates to combustible compositions that have accurately predeterminable burning rates and therefore can be advantageously used to form the timing element of a chemical time delay device employed for eX- ample as part of the fuse assembly of a demolition bomb. The present invention is concerned with a novel composition of this type and also with a novel time delay device incorporating such a composition. The time delay device and combustible composition of the present invention are particularly well suited for use in the manufacture of demolition bombs and will be illustratively described as used in such a bomb. However, as the description proceeds it will become apparent that the present composition and time delay device can be used for other purposes as well.

As is well known a demolition bomb if it is to be used effectively must normally penetrate to a substantial extent the structure or object to be destroyed. Hence it is the usual practice to incorporate in the fuse assembly of such a bomb a time delay train including a time delay device capable of providing a predetermined time interval between the time of impact and the time when the main explosive charge in the bomb is detonated. The time delay devices heretofore availa le for this purpose have been subject to a number of disadvantages both in manufacture and in operation. Thus the prior time delay devices have been relatively complicated and time-consuming to manufacture. .t has been difiicult to achieve sufficient uniformity in the mass production of such devices to provide the required accurately predeterminable time intervals. Also prior devices have been somewhat unreliable in operation, particularly under extreme temperature conditions.

It is accordingly an object of the present invention to provide a time delay device that is easier to manufacture than those previously available and which can be mass produced with a high degree of uniformity. It is another object of the invention to provide a time delay device and composition therefor that is more reliable in operation under extreme temperature conditions. Other objects of the invention will be in part obvious and in part pointed out hereafter.

As conducive to a clearer understanding of the present invention it may be pointed out that chemical time do lay devices for demolition bombs commonly comprise a metal casing having therein a passage of predetermined length that is filled with a combustible powder having a known burning rate. The time delay device is incorporated in a fuse train extending from the firing pin to the main explosive charge of the bomb in such manner that the burning time of the time delay charge essentially controls the time interval between impact and detonation of the bomb.

It has been found that the objects of the present invention can be achieved by using as a time delay charge a material that can be cast in situ in the casing of the time delay device. This material comprises in general a liquid polymer or polymer composition of a type that can be readily converted to solid form, the liquid polymer composition having dispersed therein an inorganic oxidizing agent and a metal powder or mixture of metal powders. This liquid composition is introduced into the casing of the time delay device and cured therein to form a solid, non-porous combustible time delay charge which comprises a plastic matrix having the oxidizing agent and metal powder uniformly dispersed therethrough. The charge completely fills the passage within the casing, and has a definite burning time that varies relatively little from unit to unit.

Any of various liquid plastic compositions which can be introduced into the time delay device and converted in situ into solid form, may be used as the binder in the present compositions. The preferred binders, however, are liquid polysulfide polymers of the type referred to in the specific examples given hereafter because such polymers have been found to provide a number of special advantages. Thus the polysulfide polymers are available in liquid form at a variety of different viscosities. They can be readily cured by the oxidizing action of a portion of the oxidizing material present in the composition to yield a rubbery solid that has excellent shock resistance. Moreover, the time delay compositions containing polysulfide binders have especially good low temperature properties.

The objects and advantages of the present invention can best be understood and appreciated by reference to the accompanying drawing which illustrates the tail assembly of a demolition bomb having a time delay device incorporating the present invention and bring out the relationship of the present time delay device to other portions of the bomb.

In the drawing:

FIGURE 1 is a vertical section through the tail assembly of a demolition bomb showing the general arrangement of the fuse assembly therein;

FIGURE 2 is an enlarged view of the fuse assembly showing the relative locations of the arming rotor, time delay device and firing pin;

FIGURE 3 is a perspective view of the exterior of the time delay device; and

FEGURE 4 is a central section through the time delay device taken on the line 4-4 of FIGURE 3 and showing the arrangement of the timing charge therein.

Referring to the drawing and more particularly to FIGURES l and 2 the numeral 10 designates the rear portion of the casing of a demolition bomb to which are afiixed in the usual manner the guide vanes l2. Threaded into the end of the casing 1d there is a fuse assembly generally designated by the numeral 14.

As best shown in FIGURE 2 the fuse assembly 14 comprises a casing 16 having an internal cylindrical cavity 18 formed therein in which a firing pin weight 20 is slidably mounted. The righthand end of cavity 18 is closed by a cap 22 threaded onto the casing 16 and the weight 20 is urged toward cap 22 by a helical spring 2 5 located within cavity 18. Secured to the weight 20 there is a firing pin 26 that extends through the spring 24- toward the left end of the fuse assembly.

At the left end of casing 16 there is an arming rotor 23 of conventional construction containing a detonator charge adapted to be positioned in alignment with a small passage Ed in casing 16. The passage 36 is aligned with an oval opening 32 in casing 16 which opening receives a time delay device 34.

The weight 2d has an internal threaded bore 36 which receives the threaded end of an arming vane stem 38 that extends out of the casing 16 through cap 22. At its outer (right-hand) end shaft 38 is provided with a small propeller 4t commonly called an arming vane. As shown in FIGURE 2 the stem 38 is rotatable in a cylindrical housing 42 that is threaded onto a boss 44 formed on the cap 22.

Referring now to FIGURES 3 and 4 the time delay device 34 comprises a casing 46 having a U-shaped cavity 48 therein comprising the vertical legs 50 and 52 and the transverse passage 54. Near the bottom of vertical passage 50 there is an opening 56 in casing 46- in which is located a percussion cap 58 which, when the time delay device is properly positioned in the fuse assembly, is aligned with the firing pin 26. Adjacent to the percussion cap 58 at the bottom of vertical passage 50 there is an igniter charge 60.

Near the bottom of vertical passage 52 there is a second opening 62 in casing 46, and aligned with this opening inside passage 52 there is a lead azide relay 64. Extending through the U-shaped passage 48 from the igniter charge 60 to the azide relay 64 there is a time delay charge 66, the composition and characteristics of which will be described in detail hereafter. The cavity 48 is closed by a plug or closure plate 68 fitted into the top of casing 34.

The structure so far described is essentially conventional and operates in well known manner. As the bomb is dropped the arming vanes 40 rotate to cause the arming vane stem 38 to rotate within the firing pin weight 20. When the stem 38 has rotated a predetermined number of revolutions it releases the firing pin weight 20. When the bomb strikes the structure or object to be destroyed the inertia of the firing pin weight causes it to move forward against the action of spring 24, and the firing pin '26 is driven against the cap 58 of the time delay device 34. The resulting explosion of cap 58 ignites the igniter charge 60 which in turn ignites the time delay charge 66. A predetermined time interval, say 12 to 20 seconds, elapses while the time delay charge burns through U-shaped passage 48 to the azide relay 64. At the end of this time interval the azide relay 64 is ignited and blows through passage 30 to set oif the detonator charge in the arming rotor. Referring to FIGURE 1, the detonator charge sets off a booster charge 70 located adjacent to the arming rotor 28 and booster 70 in turn sets off the main explosive charge 72 of the bomb.

As indicated above, the present invention is concerned with the time delay device 34 and particularly the nature of the time delay charge 66. In accordance with the present invention a castable time delay charge is used which can be introduced into the casing 34 in liquid form and cured therein to form a solid continuous body of material essentially composed of a plastic binder matrix having uniformly distributed therethrough a finely divided inorganic oxidizing agent and metal powder. It has been found that such a cast charge has substantial advantages over the comminuted mixtures previously used from the stand-point of both ease and economy of manufacture and accuracy and reliability in the performance of its time delay function. As indicated by the specific examples given below a variety of binders, oxidizing agents and metal powders can be used in preparing the time delay charge.

In order to point out more fully the nature of the present invention the following specific examples are given.

EXAMPLE 1 This example illustrates the use of a polysulfide polymer as a binder, potassium dichromate as the oxidizing agent and a mixture of iron and boron powders as the metal powder. The polysulfide polymer used was a liquid polythiopolymercaptan having an average molecular weight of about 1200. Such polymers may be made for example, as described in Patrick and Ferguson Patent 2,466,963. The particular polymer of the present example may be made essentially according to the procedure of Example XIII of Patent 2,466,963 except that the quantity of crosslinking agent is increased from 0.5 mol percent to 2.0 mol percent.

A castable mixture to be introduced into the time delay device 34 was prepared by thoroughly mixing 17 grams of the above-described liquid polymer with 52 grams of potassium dichromate, 1 gram of sodium tetraborate (Na B O .10H O), 20 grams of iron powder and 10 grams of boron powder. The potassium dichromate and sodium tetraborate were ground prior to mixing to a particle size such that 60% passed through a 325-mesh screen. The iron powder used was of comparable particle size and the boron powder had a nominal particle size of 1.0 to 1.5 microns. The resulting viscous mixture was deaerated under reduced pressure to remove dissolved and entrapped air.

In the present example the casing 46 of the time delay device was made of aluminum and was degreased prior to filling. The igniter charge 60 and relay 64 were then introduced into the casing and pressed into place, after which the cavity 48 was filled with the castable mixture. Care was taken to introduce the mixture into the cavity 48 in such manner as to avoid entrapment of air during the filling process. The closure 68 was then pressed in place and the unit put in an oven for curing.

Curing was effected in a forced draft circulating air oven by maintaining the delay device at a temperature of F. for a period of 24 hours. As is well-known, curing of polysulfide polymers of the type here referred to occurs as a result of oxidation of the mercaptan terminals of the polymer, and in the present composition a portion of the dichromate present performs this oxidizing function. No significant dimensional change in the time delay charge was observed during curing, and at the end of the curing period the charge had become a solid, elastic mass having the dichromate, iron and boron powders homogeneously dispersed thcrethrough.

To finish the device, the opening 56 of casing 46 was cleaned out and percussion cap 58 pressed into place,

after which the exterior of the device was coated with a Table 1 Burning Iron Boron Ratio Time in seconds Variations over a range of about 5-30 seconds can be conveniently achieved by variation of the iron-boron ratio.

EXAMPLE 2 The procedure of Example 1 was followed except that titanium powder was used in place of the mixture of iron and boron powders and the amount of sodium tetraborate was reduced. The casting mixture comprised 17.5 parts by weight of liquid polymer, 30 parts of the titanium powder, 0.5 part of sodium tetraborate and 52 parts of potassium dichromate. The average burning time for this composition was 5.2 seconds.

EXAMPLE 3 The procedure of Example 2 was followed except that the composition comprised 19.5 parts by weight of liquid polymer, 33 parts of titanium powder, 0.5 part of sodium tetraborate and 47 parts of potassium dichromate. The average burning time of this composition was 6.6 seconds.

EXAMPLE 4 EXAMPLE 5 The procedure of Example 4 was followed except that the casting composition was varied as indicated in Table 11 to give the indicated burning times.

Table 11 Liquid polymer 18. 7 18. 7 18. 7 (NHQ CMO 1.8 1.8 l. 8 0. 5 0. 5 0. 5 40. 40. 0 40. 0 p 30. O 'li powder, 30. 0 Si powder 9. 0 Burning time (sec) 8-30 I 20-30 32 EXAMPLE 6 A castable composition of the general nature described in the preceding examples was prepared using a binder of the plastisol type. To prepare the plastisol binder a comminuted polyvinyl chloride resin sold under the trade name Geon 121 was mixed with dioctylsebacate as a plasticizer and a small quantity of Thermolite 3-31 as a stabilizer. More particularly the composition comprised 9.5 parts by weight of 121, 9.5 parts dioctyl sebacate, 1.0 part Thermolite 3-31, 50 parts of potassium dichrornate and 30 parts of titanium powder.

This composition was cast in a time delay device described in Example 1 and cured in an oven at 250 F. for about 15 minutes. The burning time of units incorporating this composition was seconds.

EXAMPLE 7 The procedure of Example 6 was followed except that 40 parts of potassium dichromate and 40 parts of titanium powder were used. The burning time of the resulting composition was 5-13 seconds.

EXAMPLE 8 The procedure of Example 6 was followed except that 60 parts of potassium dichromate and 20 parts of titanium powder were used. This composition gave a burning time of -14 seconds.

EXAMPLE 9 The procedure of Example 6 was followed except that 25 parts of iron powder and 5 parts of boron powder were substituted for the 30 parts of titanium powder. The burning rate was about 27 seconds.

EXAMPLE 10 in this example the plastic binder was a polymer like that of Example 1, but having a molecular weight of 500-700. A composition was prepared comprising 18 parts by weight of this polymer, 42 parts of aluminum powder and 40 parts of potassium permanganate.

This composition when incorporated in a time delay device as in Example 1 and cured gave a burning time of about 6 seconds.

It is of course to be understood that the foregoing examples are intended to be illustrative only and that numerous changes can be made in the materials, proportions and conditions set forth in the examples without departing from the spirit of the invention as set forth in the appended claims.

We claim:

1. A time delay fuse comprising a casing having a passage of pro-determined length therein, igniting means mounted in said casing at one end of said passage, combustion relay means mounted in said casing at the other end of said passage, and a time delay fuse element substantially filling said passage between said igniting means and said relay means, said element being an elastorneric plastic matrix cast in situ in said passage and bonded to the interior wall thereof, said matrix having uniformly dispersed therethrough a solid inorganic oxidizing agent and metal powder, said plastic, oxidizing agent and metal powder being present in proportions to cause said element to be combustible.

2. A time delay fuse comprising a casing having a passage of pro-determined length therein, igniting means mounted in said casing at one end of said passage, combustion relay means mounted in said casing at the other end of said passage, and a time elay fuse element substantially filling said passage between said igniting means and said relay means, said element being a solid polysuliide polymer cast in situ in said passage and bonded to the interior Walls thereof, said polymer having uniformly dispersed therethrough an oxidizing agent which is an alkali metal dichromate and a mixture of iron and boron powders, said polymer, oxidizing agent, iron and boron being present in proportions to cause said element to be combustible.

3. A time delay fuse comprising a casing having a passage of predetermined length therein, igniting means mounted in said casing at one end of said passage, combustion relay means mounted in said casing at the other end of said passage, and a time delay fuse element substantially filling said passage between said igniting means and said relay means, said element comprising a solid polysuliide polymer cast in situ in said passage and bonded to the interior wall thereof, said polymer having uniformly dispersed therethrough a solid inorganic oxidizing agent and metal powder, said plastic, oxidizing agent and metal powder being present in proportions to cause said element to be combustible.

4. A time delay fuse according to claim 1 and wherein said metal powder is a mixture of iron and boron powders.

5. A time delay fuse according to claim 3 and wherein said metal powder is titanium powder.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A TIME DELAY FUSE COMPRISING A CASING HAVING A PASSAGE OF PRE-DETERMINED LENGTH THEREIN, IGNITING MEANS MOUNTED IN SAID CASING AT ONE END OF SAID PASSAGE, COMBUSTION RELAY MEANS MOUNTED IN SAID CASING AT THE OTHER END OF SAID PASSAGE, AND A TIME DELAY FUSE ELEMENT SUBSTANTIALLY FILLING SAID PASSAGE BETWEEN SAID IGNITING MEANS AND SAID RELAY MEANS, SAID ELEMENT BSING AN ELASTOMERIC PLASTIC MATRIX CAST IN SITU IN SAID PASSAGE AND BONDED TOTHE INTERIOR WALL THEREOF, SAID MATRIX HAVING UNIFORMLY DISPERSED THERETHROUGH A SOLID INORGANIC OXIDIZING AGENT AND METAL POWEDER, SAID PLASTIC, OXIDIZING AGENT AND 